Stirring the scraps into the muck exposes them to active methanogenic bacteria.

Cleaning up and adding water to replace what is drawn off as liquid compost.

At the output, the nitrogen-rich liquid fertilizer is extracted from the digester, which produces 5 to 10 gallons per day.

5 to 10 gallons of nitrogen-rich liquid fertilizer is produced per day.

The liquid fertilizer can be added directly to plants or covered with dirt or mulch to keep beneficial ammonia in the soil.

Now we’re cooking with (bio)gas.

Intermediate

In consuming these acids, the methanogenic bacteria raise the pH and keep it hospitable for both the acid-formers and themselves, both of which would perish if the pH dropped too low. The high-nitrogen ammonia (a byproduct of the breakdown of plant proteins) remains dissolved in the slurry, unlike in aerobic composting where it is released as a gas. Although both the acid-formers and the methanogens can suffer from rapid changes in living conditions created by the addition of feedstock, the methanogens are especially vulnerable to low pH and the introduction of too much oxygen. For this reason, biodigesters generally work on the principle of steady applications of new feedstock in regular intervals, rather than adding large amounts of biomass at once.

There are several different types of methanogenic bacteria that will colonize a digester, depending upon the slurry’s temperature range. The two ranges of interest to home-scale biodigesters are the cryophilic (50°F to 80°F) and the mesophilic (95°F to 125°F). There is a dead zone between these two temperature ranges that must be avoided. Warren’s biodigester operates in the cryophilic range. While mesophilic methanogens can break down material several times as quickly as their cryophilic counterparts, consistently maintaining high temperatures consumes a great deal of energy—which can make a net energy loss for smaller biodigesters.

The released CO2 and CH4 gases percolate through the slurry to the top of the tank. Once enough biogas accumulates, the pressure created inside the expanding rubber top reaches 0.25 to 0.5 psi, enough to move the gas through the delivery pipes and use it for cooking or lighting. As new material is added to the tank through the inlet, it displaces an equal amount of slurry through the outlet, which can be applied directly to the garden. Once applied, covering the slurry with soil helps keep the ammonia from turning to gas and losing its coveted nitrogen.

Operating the Digester

Warren’s biodigester at Maitreya primarily uses kitchen waste once in operation. Getting it up and running, however, requires a whole mess of ruminant manure—about 300 pounds’ worth. There happened to be an alpaca farm nearby, and so he used alpaca manure, but any ruminant manure will be loaded with plenty of methanogenic bacteria. He also added a few gallons of kitchen compost and tree leaves, and then filled the tank to 600 gallons.

It takes a minimum of two to three days before a biodigester begins to produce gas, since the acid-forming bacteria need to do their work before the methane-producing bacteria can go to work. About 10 to 15 pounds of kitchen scraps are collected and added to the biodigester daily, producing an average of about 70 cubic feet of biogas, which is the only source of cooking fuel for the community’s kitchen. When the methane content begins to get low, the flame will begin to burn orange. This is remedied by feeding the digester.

The scraps are shoved into the digester with a pole, and this slight agitation helps mix the slurry, exposing the material to the bacteria so it can be thoroughly composted. The tank can also be topped off with water at this time if the slurry level is low. As new feedstock is added, it displaces an equal volume of composted slurry through the outlet, which is captured in a 5-gallon bucket and then added to nearby gardens. Biodigesters prefer a carbon-to-nitrogen ratio similar to a conventional aerobic compost pile, with about 25:1 being ideal. Too much carbon-rich material (grass clippings, newspaper, etc.) will slow the digester. Manure is the most common nitrogenous material to add to rebalance a slow digester.

The biodigester at Maitreya has been operating for more than a year with consistent results. Warren and Hestia Biogas are in the final stages of getting city permits for new biodigesters, to bring an official stamp of approval to a renewable energy technology with great promise for any homestead.

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Author and builder Stephen Hren lives in Durham, North Carolina. His latest book is Tales from the Sustainable Underground: A Wild Journey with People Who Care More About the Planet Than the Law.

Comments (12)

This would be an important article even if it didn't say very much, as it is so rare for a brick and concrete type biogas plant is ever shown in the US. However, there is also plenty of explanation given as well. I was not convinced such small scale digesters could be successful in the US, but this goes a long way to proving me wrong. Great article.

If you would be seriously interested in a portable, heavy-duty poly model of these home biogas digesters available Spring 2014, please complete the brief contact form on our website here...http://hestiahomebiogas.com/home-bi...

Enjoyed article very much. Read about using a 1 kW Honda EU1000i generator for a short amount of time(dvd's). Any info for using this generator for longer periods of time possibly for heating or fridge with a 700 gallon digester? Have 1 1/2 acres plus work for a grade-school that waste unbelievable amount of food that I believe is ample to supply this digester. All the best.

Hi there. That is a super-lightweight, portable generator. While I do not know much about it other than that, I seriously doubt if it is up to snuff as anything other than an occasional-use, emergency type generator. But that is just my gut level feeling, I have no experience with it. I suggest checking out its warranty, and doing an online search to see what other users say about it.

Lovin' it Warren, just lovin' it! Your answer about safety was also great; I've been asked the same questions endlessly, particular as we were building digesters this week at the US Embassy compounds and UN compounds in Baghdad and Erbil in Kurdistan; naturally folks want to know if it is a security risk (they are banning regular compressed gas bottles in case of rocket fire from insurgents) so I take pains to explain why biogas systems of this size are actually the safest thing we can have for reliable energy while dealing with wastes. Glad to be on the global team of biogas apostles with you, brother!

How about safety? If you were to construct a biodigester for your home, are they safe? Would your neighbors have a legitimate complaint about the safety of one of these? My next question is about the smell. Would your neighbors be complaining about the smell of a biodigester? This is very intriguing.

Wow, didn't realize this article was online. This was SUCH A GREAT ARTICLE! I'm the guy who built the digester featured. Safety is not major concern, when full like the pictures there's only the equivalent of 1 liter of gasoline worth of BTUs in there. So, it's not going to blow anything up. Odor is a non-issue. If it's maintained correctly there's only a slight odor when loading that is contained within the lid. Hundreds of people walk past this one and have no idea what it is. The liquid byproduct when it comes out has a neutral, earthy smell.

We didn't have any problem with the City of Eugene with the digester, their concern was with using the imported Chinese stovetop indoors, since it wasn't UL certified. So, that's the regulatory hold-up now.

I thought Stephen and that photographer did AN AMAZING job! The layout in the print edition was just awesome. My biogas colleague Prof. Thomas Culhane, Ph.D. who also posted a comment here said Home Power is one of his favorite magazines, it was my first exposure to it. Great magazine.

I think this project is really intriguing. But there's no way my 2-person household will come up with that much compost/waste per day.
I wonder if it would be feasible to do this on an even smaller scale, or if the amount of gas produced would be moot.
Or, if we could mooch compost from friends, neighbors, restaurants, horse ranches, etc. One man's dung....

Ben, biogas is scalable all the way down to a soda bottle if needed. Many countries around the world use a 1 cubic meter unit or one made from 55-gallon drums, I went with the two cubic meter (700 gallon unit) because most American households are going to want to run a generator for a little while in addition to cooking. The 70 cubic feet of gas per day allows 3 meals per day and probably 2-3 movies on a TV/DVD set-up with a 1 kW Honda EU1000i generator.

If you feed it less there will be less gas production, but most people will be pleasantly surprised how little waste it really takes once it has a healthy culture going in it. The biodigester in the article has been functioning just fine for going on 1-1/2 years now.

A sustainable learning center I work with in Costa Rica does buy cow manure to supplement the kitchen waste and humanure that their facility generates. They are committed to reducing or eliminating their use of fossil fuels, and their purchase of local manure helps a local family generate some income while their kitchen gets more cooking gas!